Radio frequency transmit coil for magnetic resonance imaging system

ABSTRACT

A radio frequency coil is disclosed that is suitable for use with a magnetic resonance imaging apparatus. The radio frequency coil comprises first and second conductive loops connected electrically to each other by a plurality of conductive rungs. The conductive rungs each include a section that is relatively thin that will result in less attenuation to a radiation beam than other thicker sections of the rungs. Insulating regions are also disposed in areas of the radio frequency coil that are bound by adjacent rungs and the conductive loops. Portions of the insulating regions can be configured to provide a substantially similar amount of attenuation to the radiation beam as the relatively thin sections of the conductive rungs.

CROSS-REFERENCE TO RELATED APPLICATIONS

The current application is related to/claims priority under 35 U.S.C. §119(e) to U.S. patent application Ser. No. 13/796,784, filed Mar. 12,2013, the contents of which are hereby incorporated by reference in itsentirety.

TECHNICAL FIELD

The present disclosure relates generally to radio frequency (RF) coilsfor use in a magnetic resonance imaging (MRI) system, including RF coilssuitable for use with MRI guided radiation therapy systems.

BACKGROUND

Generally, in an MRI system, a substantially uniform main magneticfield, B₀, is created to cover the entire region of the subject beingimaged. The main magnetic field aligns magnetic dipoles of protonswithin the main magnetic field. Thereafter, a transient RF pulse isintroduced that causes the proton dipoles to temporarily flip. Once theRF energy from the RF pulse is removed, the dipoles flip back to theirrelaxed state and release the energy absorbed from the RF pulse in theform of a photon having some predictable radio frequency. The photonsare captured and processed to enable imaging.

Generally, the transient RF pulse is transmitted by an RF coil. One typeof RF coil commonly used in MRI is known as the “birdcage coil.” Forexample, respective examples of birdcage coils are disclosed by U.S.Pat. No. 4,680,548 to Edelstein et al., titled “Radio Frequency FieldCoil For NMR,” the entire content of which is incorporated herein byreference, and U.S. Patent Application Publication 2006/0033497 toChmielewski et al., titled “Degenerate Birdcage Coil andTransmit/Receive Apparatus and Method For Same,” the entire content ofwhich is incorporated herein by reference. Typically, a birdcage coil iscylindrical in shape and includes two conductive end loops or ringsinterconnected by an even number of rungs or axial conductors thatdivide the two end rings into arcs or segments defined therebetween.This construction gives this type of RF coil the appearance of abirdcage, and hence the name “birdcage coil.”

Notwithstanding the birdcage coils discussed above, there remains adesire for further improvements. In particular, when MRI technology isapplied to the field of radiotherapy, traditional birdcage coils are notideally suited for accommodating a radiotherapy system. U.S. Pat. No.7,907,987 to Dempsey, titled “System for delivering conformal radiationtherapy while simultaneously imaging soft tissue,” the entire content ofwhich is incorporated herein by reference, discloses an example of suchan MRI guided radiotherapy system.

SUMMARY

Disclosed herein are systems and methods for radio frequency coils foruse in a magnetic resonance imaging (MRI) system, including RF coilssuitable for use with MM guided radiation therapy systems, someembodiments of which may include a first conductive loop, a secondconductive loop and a conductive rung between the first and secondconductive loops that may be electrically connected to the first andsecond conductive loops, wherein the conductive rung may include firstand second conductive rung sections and wherein the second conductiverung section may have a thickness substantially thinner than at leastone of a thickness of the first conductive loop, a thickness of thesecond conductive loop, and a thickness of the first conductive rungsection.

In some embodiments, the second conductive rung section may have athickness that is about 5% to about 75% of the thickness of the at leastone of the first conductive loop, the second conductive loop, and thefirst conductive rung section. In other embodiments, the secondconductive rung section may have a thickness that is about 10% to about50% of the thickness of the at least one of the first conductive loop,the second conductive loop, and the first conductive rung section. Instill further embodiments, the second conductive rung section may have athickness that is about 15% to about 30% of the thickness of the atleast one of the first conductive loop, the second conductive loop, andthe first conductive rung section. The second conductive rung may alsohave a section has a thickness that is about 20% of the thickness of theat least one of the first conductive loop, the second conductive loop,and the first conductive rung section.

In certain embodiments, the conductive rung may further include a thirdconductive rung section, the second conductive rung section beingdisposed between the first and third conductive rung sections and wherethe second conductive rung section may be substantially thinner than thefirst and third conductive rung sections. In some embodiments, at leastone of the first conductive loop, the second conductive loop, and theconductive rung may include at least one of copper, silver, and aluminumor may include multiple layers of conductive materials.

In some embodiments, the radiofrequency coil may include a plurality ofconductive rungs electrically connected to the first and secondconductive loops. It may also include an insulating region disposedbetween adjacent conductive rungs and between the first and secondconductive loops and at least a portion of the insulating region mayhave a thickness selected so that the portion of the insulating regionand the second conductive rung section both provide substantially thesame amount of attenuation to a radiation beam. In some embodiments, theinsulating region may be a polyimide.

In further embodiments the radio frequency coils may also include aprinted circuit board (PCB) substrate and the conductive rung mayinclude a layer of conductive material formed on a first side of the PCBsubstrate. The coil may also include an insulating region disposedadjacent to the conductive rung and between the first and secondconductive loops, where the insulating region may include a firstinsulating layer formed on the first side of the PCB substrate and asecond insulating layer formed on a second side of the PCB substrate.

The radio frequency coil may also include PIN diode circuitry locatedadjacent the first and third conductive rung sections, and the magneticresonance imaging apparatus may have a field strength less than 1.0 T.

These and other features, aspects, and advantages of the presentdisclosure will become better understood with reference to the followingdescription and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, aspects, and embodiments are described in conjunction with theattached drawings, in which:

FIG. 1 shows a perspective view of an MRI system;

FIG. 2 shows a simplified cross-sectional view of the MRI system shownin FIG. 1;

FIG. 3A shows a perspective view of an RF coil according to the presentdisclosure;

FIG. 3B shows a plan view of the RF coil shown in FIG. 3A where the RFcoil is unrolled;

FIG. 4 shows a side view of an embodiment of a conductive rung that canbe used with the RF coil shown in FIGS. 3A and 3B; and

FIG. 5 shows a cross-sectional view of a portion of the RF coil takenalong section line V-V in FIG. 3B.

DETAILED DESCRIPTION

The following description illustrates embodiments by way of example andnot by way of limitation. All numbers disclosed herein are approximatevalues unless stated otherwise, regardless whether the word “about” or“approximately” is used in connection therewith. Whenever a numericalrange with a lower limit and an upper limit is disclosed, any numberfalling within the range is specifically and expressly disclosed.

The RF coil assembly concepts of the present disclosure may be used withany type of magnetic resonance imaging (MRI) system. It is particularlywell suited for use with a split solenoid or horizontal “open” MRI thatincludes a gap between two horizontal MRI magnet halves. The RF coilassemblies disclosed herein are further well suited for use with ahorizontal open MRI that is used with an additional instrument beingoperated within its gap. FIG. 1 depicts such an arrangement with ahorizontal open MRI 10 having first and second main magnet housings 11 aand 11 b separated by a gap region 12. An instrument 14 is mounted inthe gap region 12 on a gantry 16. Also depicted are a patient 18 andpatient couch 20. In some embodiments, the gantry 16 can be used toreposition the instrument 14 about the patient 18 (i.e., about theZ-axis shown in FIG. 1).

The embodiment of FIG. 1 can include elements of a system of theassignee of the current application, ViewRay, Incorporated, described inpart in U.S. Pat. No. 7,907,987 to Dempsey, titled “System forDelivering Conformal Radiation Therapy while Simultaneously Imaging SoftTissue” (hereafter “Dempsey '987”), which is hereby incorporated byreference. For example, the instrument 14 can comprise a radiationtherapy device and associated multi-leaf collimator (MLC), which, incombination with a fast-imaging horizontal open MRI, allows for improvedradiation therapy that can account for a target's location duringradiation treatment, as discussed in Dempsey '987. While only a singleassembly is shown as the instrument 14 in FIG. 1, some embodiments caninclude multiple assemblies associated with instrument 14, e.g.,multiple radiation emitters and/or MLC devices. For example, someembodiments may include three radiation head assemblies (not shown inFIG. 1) mounted in gap 12, distributed about the Z-axis, and rotatableabout the Z-axis on the gantry 16. While some aspects of the embodimentsdisclosed herein are described with respect to the system disclosed byDempsey '987, such aspects are not required for use with the disclosedRF coil assembly. It is contemplated that the RF coil assembly disclosedherein may be used in any type of MRI, with or without the use of anassociated instrument 14. Furthermore, for systems utilizing aninstrument 14, such instruments are not limited to radiation therapydevices such as radiation sources or linear particle accelerators(LINACs), but can include any type of instrument used with an MRI.

FIG. 2 is diagrammatic cross-section of the system shown in FIG. 1. Theembodiment of FIG. 2 depicts the horizontal open MRI 10 including a pairof main magnets 22 a and 22 b, separated by the gap 12. The MRI 10 canbe used to image a region of interest 24 above the patient couch 20,while the instrument 14 can be used to emit radiation 15 forsimultaneously performing some form of treatment to the patient withinthe region of interest 24. The MRI 10 also includes an RF transmit coilassembly 100 that extends across the gap 12. Embodiments of the RF coilassembly 100 are described in greater detail below. The MRI 10 caninclude additional conventional components not shown, for example,gradient coils and potentially one or more shim coils. The coordinatesystem used in the figures and throughout this disclosure refers to thelongitudinal axis through the MRI bore as the Z-axis. The X-axis extendsperpendicular to the Z-axis and from side to side of the MRI 10; theY-axis extends perpendicular to the Z-axis and from the bottom to thetop of MRI 10.

As shown in FIG. 2, the RF coil assembly 100 extends between theinstrument 14 and the region of interest 24. So, for example, inembodiments where the instrument 14 comprises a radiation emittingdevice such as those used with a radiation therapy system, a portion ofthe RF coil assembly will be in the path of radiation 15 that is beingdirected from the instrument 14 towards the patient at the region ofinterest 24. Simply inserting a conventional RF coil assembly in such aposition poses problems, both for MRI operation and for operation of theradiation therapy device as well as for other systems that could beimplemented as the instrument 14. For example, a conventional RFtransmit coil includes structure that would interfere with a radiationtherapy beam passing therethrough from the instrument 14, potentiallyattenuating the beam to the point where the RF coil degrades the qualityof the treatment to the point of it being clinically unacceptable. Thus,an RF coil is disclosed herein that allows for proper MRI imagingwithout interfering with the operation of an instrument 14. For example,embodiments of the RF coil disclosed herein can allow for proper MRIimaging without causing an undesirable level of attenuation to aradiation beam being emitted from instrument 14 through a portion of theRF coil.

FIG. 3A shows an embodiment of such an RF coil, which is designated asRF coil 100. FIG. 3B shows a plan view of RF coil 100 opened and laidflat, i.e., on a planar surface. FIG. 3A shows the configuration of RFcoil 100 as installed in MRI system 10. When installed, the RF coil 100can define an inner space having a generally cylindrical shape, thelength of which extends parallel to the Z axis of the MRI system 10 asshown in FIG. 3A.

The RF coil 100 comprises a first conductive loop 110 and a secondconductive loop 120, both of which are coaxial with the Z-axis. Thefirst and second conductive loops 110 and 120 are electrically connectedto each other via a plurality of conductive rungs 150, each of whichextend at least somewhat parallel to the Z-axis between the first andsecond conductive loops 110 and 120.

Although the RF coil 100 is shown in FIGS. 3A and 3B as having sixteenconductive rungs 150, alternative embodiments of the RF coil 100 canhave other numbers of rungs. For example, exemplary embodiments of theRF coil 100 can include numbers of conductive rungs 150 equal to anymultiple of four. However, embodiments that include fewer than sixteenconductive rungs 150 may have less than desirable RF emissionuniformity.

Each of the plurality of conductive rungs 150 comprises a first endsection 151 a, a middle section 152, and a second end section 151 b,disposed in series with each other between the first conductive loop 110and the second conductive loop 120. The first end section 151 a iselectrically connected to the first conductive loop 110. The second endsection 151 b is electrically connected to the second conductive loop120. The middle section 152 is electrically connected to the first endsection 151 a and to the second end section 151 b. Thus, an electricalcurrent can flow between the first conductive loop 110 and the secondconductive loop 120 through the first end section 151 a, the middlesection 152, and the second end section 151 b of the conductive rung150.

Insulating regions 180 are defined by respective pairs of adjacent rungs150 and the first and second conductive loops 110 and 120. Theinsulating regions 180 may comprise electrically insulating materials asdiscussed in more detail below.

In one embodiment of the disclosed systems, PIN diode decouplingcircuitry is located outside the path of the radiation beam, adjacentfirst end section 151 a and second end section 151 b. This embodiment isespecially effective in conjunction with low field MRI (for example,less than a 1.0 T field strength). In one implementation, a coax cableis used to provide the off-center location of the PIN diode decoupling.Similar more efficient methods can also be implemented such as utilizingtwisted pairs or wide parallel conductors with self-canceling fieldprofiles. The tuning capacitors may be placed in the rung and ring gapsoutside of the radiation beam, and may be reduced to compensate forincreased inductance due to the conductor length change.

FIG. 4 shows a block diagram illustrating a generalized side view ofconductive portions of an exemplary conductive rung 150. Note that FIG.4 does not show insulating portions of the conductive rung 150 so thatthe conductive regions of the conductive rung 150 can be more clearlyillustrated. Also, the diagram shown in FIG. 4 is not necessarily drawnto scale, and is not intended to be limiting as to the exact shapes ofthe conductive regions of the conductive rung 150. Rather, FIG. 4 ismerely provided to illustrate how the conductive rung 150 can includerelatively thinner and thicker conductive portions. Thus, as shown inFIG. 4, the conductive rungs 150 include a relatively thin conductingportion, i.e., at least some conductive portion of each conductive rung150 can have a thickness that is substantially thinner than a thickness(or thicknesses) of other conductive portions of the conductive rung150. More specifically, the middle section 152 includes a conductiveportion that is substantially thinner than conductive portions of thefirst and second end sections 151 a and 151 b. Also, in suchembodiments, the conductive portion of the middle section 152 can besubstantially thinner than conductive portions of the first and secondconductive loops 110 and 120.

The conductive portion of the middle section 152 can have a thicknessthat is about 5% to about 75% of the thickness of the conductiveportions of the first and second end sections 151 a and 151 b. In someembodiments, the conductive portion of the middle section 152 can have athickness that is about 10% to about 50% of the thickness of conductiveportions of the first and second end sections 151 a and 151 b. In someembodiments, the conductive portion of the middle section 152 can have athickness that is about 15% to about 30% of the thickness of conductiveportions of the first and second end sections 151 a and 151 b. In someembodiments, the conductive portion of the middle section 152 can have athickness that is about 20% of the thickness of conductive portions ofthe first and second end sections 151 a and 151 b.

The conductive portions of the first conductive loop 110, the secondconductive loop 120, and the plurality of conductive rungs 150 cancomprise one or more of many different conductive materials known to besuitable for the construction of MRI RF coils. For example, conductiveportions of the conductive loops 110 and 120 and the conductive rungs150 can comprise one or more of copper, silver, and/or aluminum. Also,in some embodiments, one or more of the first conductive loop 110, thesecond conductive loop 120, and the conductive rungs 150 can be formedof laminated layers, which can include one or more layers of conductivematerials, such as copper, silver, and/or aluminum.

Some embodiments can include conductive portions having a thickness thatyields minimal loss. For example, a desirable thickness of copper thatwould yield minimal loss is about 10 skin depths, where a skin depth canbe calculated according to the following expression:

${{{Skin}\mspace{14mu}{Depth}} = \delta},{= \sqrt{\frac{2\rho}{2\pi\; f\;\mu_{0}\mu_{R}}}}$where: ρ = bulk  resitivity(ohm − meters) f = frequency(Hertz)μ₀ = permeability  constant(Henries/meter) = 4π × 10⁻⁷μ_(r) = relative  permeability(usually ∼ 1)Thus, for example, at 14.7 MHz, with copper being the conductivematerial, 10 skin depths would be approximately equal to 0.172 mm.However, a layer of copper having a thickness of 0.172 mm would cause anamount of attenuation to radiation beam 15 that is approximatelyequivalent to about 1.53 mm of water (Calculation: 0.172 mm*8.9 (densityof copper/density of water)=1.53 mm), which is an undesirable amount ofattenuation. But by reducing the copper thickness in path of theradiation beam 15 by a significant factor, the attenuation can bereduced to a satisfactory level. For example, if the thickness of thecopper in the path of radiation beam 15 is reduced by a factor of five,the attenuation caused by the copper can be reduced to an amountapproximately equivalent to about 0.3 mm of water (Calculation: 0.03302mm*8.9=0.3 mm).

Thus, referring back to FIG. 2, preferably the relatively thinner middlesection 152 is provided in the path of radiation 15 emitted from theinstrument 14 during operation. One advantage of constructing the middlesections 152 of the rungs 150 to have a relatively thinner conductor isthat the thinner conductor causes less attenuation to the radiation 15than a thicker conductor, such as the thicker conductors used to formthe first and second end sections 151 a and 151 b. The radiation 15 canthus penetrate the middle section 152 of the RF coil 100, reach thepatient 18, and still be suitable for exerting the desired level ofradiation therapy on the region of interest 24 of the patient 18.

As will be appreciated by those skilled in the art, reducing thethickness of the conductive portions of the rungs of a conventionalbirdcage-style RF coil has the effect of raising the RF resistance ofthe rungs. An important performance factor of RF coils is the qualityfactor (Q-factor) of the coil, which should be maximized. For abirdcage-style RF coil, the Q-factor is inversely proportional to the RFresistance of the coil. Thus, thinning the rungs of a conventional RFcoil in order to avoid attenuating a radiation beam 15 has theundesirable effect of increasing the RF resistance of the coil andtherefore reducing the Q-factor of the coil.

The present disclosure provides a solution to this problem by providingrungs 150 that are only partially thinned. The RF coil described hereinincludes the relatively thin middle sections 152 in combination withrelatively thick rung end sections 151 a and 151 b and conductive loops110 and 120. Aspects of the present disclosure are based on anobservation that the amount of RF current in the conductive loops 110and 120 will be significantly higher than the amount of RF current ineach of the rungs 150. For example, the amount of RF current in theconductive loops 110 and 120 may be four to five times higher than theamount of RF current in each of the rungs 150. Therefore, the increasedRF resistance caused by thinning portions of the rungs 150 can besignificantly compensated for by reducing the RF resistance of theconductive loops 110 and 120, as well as by reducing the RF resistanceof significant portions of the rungs 150 (i.e., the rung end sections151 a and 151 b). The result is an acceptably small amount ofdegradation to the Q-factor or performance of the RF coil 100.

Referring again to FIG. 3B, in some embodiments, the RF coil 100 can beconstructed using the thin printed circuit board (PCB) technology. Insuch embodiments, multiple thicknesses of conductive layers can beapplied to one or both sides of a PCB 190 (not shown in FIG. 3A forclarity purposes). For embodiments where conductive layers are appliedto both sides of PCB 190, vias can be used to electrically connect theconductive layers that are located on opposing sides of the PCB 190. Insome embodiments, for example, the conductive portions of the middlesections 152 of the rungs 150 can be located on one side (top) of thePCB 190, and the end sections 151 a and 151 b, the first end loop 110,and the second end loop 120 can all be located on the other side(bottom) of the PCB 190. In such embodiments, vias can be used toelectrically connect the middle sections 152 to adjacent end sections151 a and 151 b through the PCB 190.

The RF coil 100 can be configured as a band-pass, low-pass, or high-passcoil, for example by including circuit elements, such as capacitors, inthe rungs 150 and/or insulating regions 180 according to known coildesign and tuning methods. Also, slots in the conductive layers can beprovided for reducing eddy currents.

While the RF coil 100 has been described as being constructed using PCBtechnology, alternative embodiments of the RF coil 100 can includealternative methods of construction that do not involve PCB technology.For example, thin strips of conductive material can be applied to aformer made of insulating material in order to construct the middlesections 152 of the rungs 150. The end loops 110 and 120, as well as theend sections 151 a and 151 b of the rungs 150, can be constructed ofthicker conductive materials, layers, or groups of layers, that can besoldered or otherwise connected to the thin conductive strips of themiddle sections 152 of the rungs 150. In some embodiments, desiredthicknesses of the conductive materials can be achieved by selectivelythickening the conductive materials using known plating processes orother known construction techniques.

The thickness of the insulating regions 180 can also vary, for examplein order to allow for uniform attenuation of radiation beam 15. Theinsulating regions 180 can be formed of single or multiple layers ofinsulating materials, which can include one or more different insulatingmaterials, such as polyimide film (e.g., KAPTON® polyimide film,available from DuPont, Wilmington, Del.).

FIG. 5 shows an exemplary cross-sectional view taken along section linesV-V shown in FIG. 3B. The section shown in FIG. 5 illustrates portionsof the RF coil 100 that could be positioned in the path of the radiationbeam 15 depending on the position of the instrument 14 relative to theRF coil 100. As shown in FIG. 5, the RF coil 100 includes anon-conductive (insulating) PCB substrate 190 having a thickness T1.Insulating layers 192 a and 192 b having a thickness T2 are formed onthe PCB substrate 190 in the insulating regions 180. A conducting layer194 having a thickness T3 is formed on the PCB substrate 190 in the rung150 region. Also, additional insulating layers 196 a and 196 b having athickness T4 are formed on an opposing side of the PCB substrate 190 inthe insulating regions 180. Because the material of conductive layer 194causes greater radiation attenuation than the substrate and insulatingmaterials, the total thickness T1+T2+T3 of the substrate 190 andinsulating layers 192 and 196 can be greater than the thickness T1+T3 ofthe substrate 190 and conductive layer 194 in order to provide foruniform attenuation to the radiation beam 15. For example, in someembodiments, the PCB substrate 190 can have a thickness T1=0.0762 mm,the insulating layers 192 a and 192 b can each have a thicknessT2=0.0762 mm, the conducting layer 194 can have a thickness T3=0.03302mm, and the insulating layers 196 a and 196 b can each have a thicknessT4=0.127 mm. In such embodiments, the PCB substrate 190, and theinsulating layers 192 and 196 can be formed of KAPTON® polyimide film,and the conducting layer 194 can be formed of copper. Other materialsnot shown in FIG. 5 that may be included in an RF coil 100, such asadhesives that may be used for securing the layers 192, 194, and 196 tothe substrate 190, can be taken into consideration when determiningappropriate thicknesses of the layers 192 and 196 for providingsubstantially the same attenuation as the conducting layer 194.

While each of the layers 192, 194, and 196 is shown as a single layer,alternatively the blocks 192, 194, and/or 196 can be formed of one ormore actual layers of material. Also, one or more of the layers 192,194, and/or 196 can include one or more different materials.

Although the illustrative embodiments of the present disclosure havebeen described herein with reference to the accompanying drawings andexamples, it is to be understood that the disclosure is not limited tothose precise embodiments, and various other changes and modificationsmay be affected therein by one skilled in the art without departing fromthe scope of spirit of the disclosure. All such changes andmodifications are intended to be included within the scope of thedisclosure as defined by the appended claims.

What is claimed is:
 1. A radio frequency coil for use with a magneticresonance imaging apparatus, the radio frequency coil comprising: afirst conductive loop; a second conductive loop; and a conductive rungdisposed between the first and second conductive loops and electricallyconnected to the first and second conductive loops, wherein theconductive rung includes a first conductive rung section and a secondconductive rung section, and wherein the second conductive rung sectionhas a thickness that is thinner than a thickness of the first conductiverung section.
 2. The radio frequency coil of claim 1, wherein the secondconductive rung section has a thickness that is about 5% to about 75% ofthe thickness of the at least one of the first conductive loop, thesecond conductive loop, and the first conductive rung section.
 3. Theradio frequency coil of claim 2, wherein the second conductive rungsection has a thickness that is about 10% to about 50% of the thicknessof the at least one of the first conductive loop, the second conductiveloop, and the first conductive rung section.
 4. The radio frequency coilof claim 3, wherein the second conductive rung section has a thicknessthat is about 15% to about 30% of the thickness of the at least one ofthe first conductive loop, the second conductive loop, and the firstconductive rung section.
 5. The radio frequency coil of claim 4, whereinthe second conductive rung section has a thickness that is about 20% ofthe thickness of the at least one of the first conductive loop, thesecond conductive loop, and the first conductive rung section.
 6. Theradio frequency coil of claim 1, wherein the conductive rung furtherincludes a third conductive rung section, the second conductive rungsection being disposed between the first and third conductive rungsections.
 7. The radio frequency coil of claim 6, wherein the secondconductive rung section is substantially thinner than the first andthird conductive rung sections.
 8. The radio frequency coil of claim 6,further comprising PIN diode circuitry located adjacent the first andthird conductive rung sections.
 9. The radio frequency coil of claim 8,wherein the magnetic resonance imaging apparatus has a field strengthless than 1.0T.
 10. The radio frequency coil of claim 1, wherein thesecond conductive rung section has a thickness that is thinner than atleast one of a thickness of the first conductive loop or a thickness ofthe second conductive loop.
 11. The radio frequency coil of claim 1,wherein a radiation beam emitted along a second path through the secondconductive rung section has a reduced attenuation compared to along afirst path through the first conductive rung section.
 12. The radiofrequency coil of claim 1, comprising a plurality of conductive rungs,the plurality of conductive rungs including said conductive rung, eachof the plurality of conductive rungs being electrically connected to thefirst and second conductive loops.
 13. The radio frequency coil of claim12, further comprising an insulating region disposed between adjacentconductive rungs and between the first and second conductive loops. 14.The radio frequency coil of claim 13, wherein at least a portion of theinsulating region has a thickness that is selected such that the portionof the insulating region and the second conductive rung section bothprovide substantially the same amount of attenuation to a radiationbeam.
 15. The radio frequency coil of claim 13, wherein the insulatingregion comprises polyimide.
 16. The radio frequency coil of claim 1,further comprising a printed circuit board (PCB) substrate, wherein theconductive rung includes a layer of conductive material formed on afirst side of the PCB substrate.
 17. The radio frequency coil of claim16, further comprising an insulating region disposed adjacent to theconductive rung and between the first and second conductive loops,wherein the insulating region comprises a first insulating layer formedon the first side of the PCB substrate and a second insulating layerformed on a second side of the PCB substrate.
 18. A radio frequency coilfor use with a magnetic resonance imaging apparatus, the radio frequencycoil comprising: a first conductive loop; a second conductive loop; anda conductive rung disposed between the first and second conductive loopsand electrically connected to the first and second conductive loops,wherein the conductive rung includes a first conductive rung section anda second conductive rung section, and wherein the second conductive rungsection has a thickness that is less than a thickness of the firstconductive rung section.
 19. The radio frequency coil of claim 18,wherein the thickness of the second conductive rung section is about 20%of the thickness of the at least one of the first conductive loop, thesecond conductive loop, and the first conductive rung section.
 20. Theradio frequency coil of claim 18, wherein the conductive rung furtherincludes a third conductive rung section, the second conductive rungsection being disposed between the first and third conductive rungsections, wherein the thickness of the second conductive rung section issubstantially less than respective thicknesses of the first and thirdconductive rung sections.
 21. The radio frequency coil of claim 18, thesecond conductive rung section has a thickness that is thinner than atleast one of a thickness of the first conductive loop or a thickness ofthe second conductive loop.
 22. The radio frequency coil of claim 18,wherein a radiation beam emitted along a second path through the secondconductive rung section has a reduced attenuation compared to along afirst path through the first conductive rung section.